A conventional current-resonant switching power supply is shown in FIG. 1. In FIG. 1, a full-wave rectifier 2 (corresponding to an input rectifier circuit) rectifies an AC voltage from an AC power source 1 and outputs a full-wave current-voltage to a smoothing capacitor 3. The smoothing capacitor 3 smoothes the full-wave current-voltage of the full-wave rectifier circuit 2.
A series circuit of a first switching element Q1 formed by a MOSFET or the like and a second switching element Q2 formed by the MOSFET or the like is connected to the opposite ends of the smoothing capacitor 3. A rectifier (diode) 6 is connected in parallel to the first switching element Q1, and a rectifier 7 is connected in parallel to the second switching element Q2. A voltage resonant capacitor Crv is connected in parallel to the first switching element Q1.
A series circuit of a current resonant capacitor Cri, a reactor Lr, and a primary winding P1 of a resonant transformer T is connected in parallel to the first switching element Q1. A resonant circuit is formed with the voltage resonant capacitor Crv, the current resonant capacitor Cri, the resonant reactor Lr, and the primary winding P1 of the resonant transformer T.
The primary winding P1 and a secondary winding S of the resonant transformer T are wound so that a common mode voltage is generated respectively, and a rectifying and smoothing circuit formed with a rectifier D0 and a smoothing capacitor 14 is connected to the secondary winding S of the resonant transformer T. The rectifying and smoothing circuit rectifies and smoothes a voltage (ON/OFF controlled pulse voltage) induced by the secondary winding S of the resonant transformer T and outputs a DC output to a load 16.
A voltage detection circuit 15 is connected to the opposite ends of the smoothing capacitor 14, to detect an output voltage of the smoothing capacitor 14, and outputs the detected voltage to a control circuit 11a. The control circuit 11a controls the voltage of the load 16 to maintain a constant voltage by turning on and off alternately the first switching element Q1 and the second switching element Q2 according to PWN control, based on the detected voltage from the voltage detection circuit 15. In this case, the first switching element Q1 and the second switching element Q2 are alternately turned on and off by applying the voltage to respective gates of the first switching element Q1 and the second switching element Q2.
The operation of the thus formed conventional resonant switching power supply is explained with reference to timing charts in FIGS. 2 to 4. FIG. 2 is a timing chart of a signal in respective units of the switching power supply. FIG. 3 shows a timing chart of the respective signals indicating in detail a period during which energy is transmitted from the primary side to the secondary side of the resonant transformer in the switching power supply. FIG. 4 is a detailed timing chart in periods T1 to T10 of the signal in the respective units of the switching power supply.
In FIGS. 2 to 4, reference sign IP1 denotes a current flowing to the primary winding P1, VQ1 denotes both terminal voltage of the first switching element Q1, IQ1 denotes a current flowing to the first switching element Q1, VQ2 denotes both terminal voltage of the second switching element Q2, IQ2 denotes a current flowing to the second switching element Q2, VD0 denotes a voltage of the rectifier D0, ID0 denotes a current flowing to the rectifier D0, VP1 denotes both terminal voltage of the primary winding P1, and VS denotes a both terminal voltage of the secondary winding S. The resonant reactor Lr is sufficiently smaller than an exciting inductance Lp of the primary winding P1, and the voltage resonant capacitor Crv is sufficiently smaller than the current resonant capacitor Cri.
In the period T1, the first switching element Q1 is turned OFF, and the second switching element Q2 has just been changed from ON to OFF. A resonance current IP1 passes through a path extending along the resonant reactor Lr, the primary winding P1, the voltage resonant capacitor Crv, the current resonant capacitor Cri, and the resonant reactor Lr by the energy stored in the resonant reactor Lr and the exciting inductance Lp of the resonant transformer T. The voltage resonant capacitor Crv discharges due to the resonance of the exciting inductance Lp of the resonant transformer T, the resonant reactor Lr, and the voltage resonant capacitor Crv, and hence, the voltage VQ1 drops and the voltage VQ2 rises.
In the period T2, both the first switching element Q1 and the second switching element Q2 are turned OFF. The voltage resonant capacitor Crv finishes discharge, and the voltage VQ1 is zero and the voltage VQ2 is equal to the both terminal voltage of the smoothing capacitor 3. The resonance current IP1 continues to pass through a path extending along the resonant reactor Lr, the primary winding P1, the rectifier 6, the current resonant capacitor Cri, and the resonant reactor Lr. At this time, the first switching element Q1 is turned ON, and phase shifts to the period T3. In the period T3, the first switching element Q1 is turned ON, and the second switching element Q2 is turned OFF.
The current IP1 continues to pass through a path extending along the resonant reactor Lr, the primary winding P1, the first switching element Q1, the current resonant capacitor Cri, and the resonant reactor Lr while decreasing, and when the current becomes zero, the phase shifts to the period T4.
In the period T4, the first switching element Q1 is turned ON, and the second switching element Q2 is OFF. The flow direction of current IP1 is reversed, and the resonance current passes through a path extending along the primary winding P1, the resonant reactor Lr, the current resonant capacitor Cri, the first switching element Q1, and the primary winding P1, and magnetic flux of the transformer T is reset.
In the periods T1 to T4, the current IP1 and the current IQ1 flow due to the resonance of the exciting inductance Lp of the resonant transformer T, the resonant reactor Lr, and the current resonant capacitor Cri.
In the period T5, the first switching element Q1 is OFF and the second switching element Q2 is OFF. The current IP1 passes through a path extending along the primary winding P1, the resonant reactor Lr, the current resonant capacitor Cri, the voltage resonant capacitor Crv, and the primary winding P1. The current resonant capacitor Crv is charged due to the resonance of the exciting inductance Lp of the resonant transformer T, the resonant reactor Lr, and the voltage resonant capacitor Crv, and the voltage VQ1 rises and the voltage VQ2 drops.
In the period T6, both the first switching element Q1 and the second switching element Q2 are OFF. The voltage resonant capacitor Crv is charged up to the voltage of the smoothing capacitor 3, so that the voltage VQ1 becomes equal to the voltage of the smoothing capacitor 3 and the voltage VQ2 becomes zero. The resonance current IP1 continues to pass through a path extending along the primary winding P1, the resonant reactor Lr, the current resonant capacitor Cri, the smoothing capacitor 3, the rectifier 7, and the primary winding P1. In the period T7, the second switching element Q2 is turned ON, and the first switching element Q1 is still OFF. The resonance current continues to pass through a path extending along the primary winding P1, the resonant reactor Lr, the current resonant capacitor Cri, the smoothing capacitor 3, the second switching element Q2, and the primary winding P1. In the periods T5 to T7, the current IP1 and the current IQ2 flow due to the resonance of the exciting inductance Lp of the resonant transformer T, the resonant reactor Lr, and the current resonant capacitor Cri.
In the period T8, the second switching element Q2 is turned ON and the first switching element Q1 is OFF. The resonance current IP1 and the current IQ2 pass through a path extending along the second switching element Q2, the primary winding P1, the resonant reactor Lr, and the current resonant capacitor Cri, and the current ID0 starts to flow to the secondary side rectifier D0. In the period T9, the second switching element Q2 is ON and the first switching element Q1 is OFF. The current IP1 and the current IQ2 pass through a path extending along the current resonant capacitor Cri, the resonant reactor Lr, the primary winding P1, and the second switching element Q2.
In the periods T8 and T9, the current IP1 and the current IQ2 flow due to the resonance of the resonant reactor Lr and the current resonant capacitor Cri. In the periods T8 and T9, energy is transmitted from the primary winding P1 to the secondary winding S of the resonant transformer T. At this time, the current ID0 transmitted from the primary side to the secondary side rises while drawing an arc, and at a certain point, starts to drop and becomes zero when a resonance period t1 (corresponding to the periods T6 to T9) has passed. The energy transmitted to the secondary side is rectified and smoothed by the rectifier D0 and the smoothing capacitor 14, and the DC power is supplied to the load 16.
In the period T10, the second switching element Q2 keeps the ON state, and the current IP1 and the current IQ2 pass through a path extending along the current resonant capacitor Cri, the resonant reactor Lr, the primary winding P1, and the second switching element Q2, but the current ID0 stops to flow. In the period T10, the current IP1 and the current IQ2 flow due to the resonance of the exciting inductance Lp of the resonant transformer T, the resonant reactor Lr, and the current resonant capacitor Cri. When the period T10 (corresponding to the period t2 decided based on an oscillating frequency or a duty ratio) has passed, the second switching element Q2 is turned OFF and the first switching element Q1 is turned ON, and the phase shifts to a reset period t3 (corresponding to the periods T1 to T5).